High power Q-switched pulsed CO2 lasers have been manufactured in folded waveguide and folded free space Gaussian mode designs. Such a Q-switched CO2 lasers have optical characteristics attractive for a number of specialized, high-value applications in materials processing, such as via hole drilling, and printed circuit board (PCB) polymer ablation. A high-power Q-switched CO2 laser is also useful as an oscillator in a master-oscillator power-amplifier (MOPA) arrangement for use in plasma-EUV (extreme ultraviolet) radiation generation for photolithography. Such a laser combines very high peak instantaneous optical power, for example, about 10 kilowatts (kW) or greater, with modest average power, for example, about 10 watts (W) or greater, in a compact package.
One common method of Q-switching a high power CO2 laser involves use of an intra-cavity electro-optic (E-O) modulator. This provides very fast, for example, tens of nanoseconds (ns), optical switching, resulting in a laser with short optical pulses, for example about 100 ns FWHM (full width at half maximum), and high pulse repetition rate, for example about 100 kilohertz (kHz) or greater. To date, the only material with a suitable combination of high electro-optic coefficient, high bulk resistivity, and low infrared absorption is single-crystal cadmium telluride (CdTe). A pulsed CO2 laser including a CdTe E-O Q-switch is described in U.S. Pat. No. 6,826,204 assigned to the assignee of the present invention.
A complete reliance on CdTe for E-O Q-switching CO2 lasers presents two key issues. The first is high component costs which include the cost of the CdTe crystal itself and the cost of a high-speed, high-voltage modulator driver for driving the CdTe crystal. These component costs result in a cost of the finished pulsed CO2 laser many times higher than that of a continuous wave (CW) CO2 laser of comparable average power.
More significant, however, is that there is only a very limited supply of modulator quality CdTe crystals. For many years there has been only one vendor (Keystone Crystals Corporation, of Butler, Pa.) for such CdTe crystals. Other crystal-growers have attempted to grow CdTe crystals of the required quality but those attempts have been generally unsuccessful. This means in effect that there may be a long-term instability of supply for E-O switch quality CdTe crystals. This is troublesome when considering development of a large-scale commercial product. The high cost associated with problems in growing the CdTe crystals presents a significant barrier to further development of E-O Q-switched CO2 lasers and applying such lasers in material processing systems.
An alternate, relatively inexpensive approach to Q-switching a CO2 laser involves scanning a resonator end-mirror or a resonator fold-mirror such that the mirror sweeps reciprocally from one completely misaligned position to another through an optimally aligned position. This approach is described in U.S. patent application Ser. No. 11/638,645, filed Mar. 13, 2006, assigned to the assignee of the present invention and incorporated herein by reference. A problem with this approach is that the mirrors are preferably scanned at a characteristic resonant frequency to provide a suitable combination of scan-angle and sweep-speed. This limits the range of PRF available in such a laser, and accordingly, such a laser cannot be expected to have as flexible operating parameters as an E-O Q-switched CO2 laser. There is a need for a pulsed Q-switched laser that does not require a CdTe E-O Q-switch, but that can be Q-switched at comparable rates and with the same flexibility as a CdTe E-O Q-switched CO2 laser.